The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting 1 or 2 from a given number. Students may use the structure of the count sequence and count back to determine the difference (MP7). These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to subtract other numbers within 10.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Share and record responses.
Keep expressions and work displayed.
Repeat with each expression.
Find the value of each expression mentally.
Student Response
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Advancing Student Thinking
Activity Synthesis
“How are the last 2 expressions alike? How are they different?” (They both start with 8. They are both subtraction. One subtracts 1 and the other subtracts 2. One is 7 and the other is 6.)
Activity 1
Standards Alignment
Building On
Addressing
1.OA.C.5
Relate counting to addition and subtraction (e.g., by counting on 2 to add 2).
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., ); decomposing a number leading to a ten (e.g., ); using the relationship between addition and subtraction (e.g., knowing that , one knows ); and creating equivalent but easier or known sums (e.g., adding by creating the known equivalent ).
The purpose of this activity is for students to learn Stage 2 of the Check It Off center. Students are introduced to the term “difference” as they subtract within 10. They take turns picking two number cards and finding the difference of those two numbers in any way that makes sense to them. Students may count all and remove some before counting again, count back, or use a known fact. Students check off the difference on their recording sheet and fill in the subtraction expression that represents the value of the difference. The partner who checks off the most numbers wins.
If students write an expression with the minuend and subtrahend in reversed order, for example instead of , ask them to represent the expression with cubes or two-color counters. Students will see that if they have 4 cubes, they are not able to take away 7. Allow students to think about how they can write an expression that represents beginning with one of the numbers and taking away the other to find the difference. Avoid telling students that the greater number is always first in a subtraction expression since this will not remain true when students begin working with negative numbers in a later grade. Note that students should not be expected to know or use the terms “minuend” or “subtrahend.”
MLR8 Discussion Supports. Synthesis: Invite students to use the following sentence frames: “The difference is . . .” and “To find the difference I can . . . .”
Advances: Speaking, Representing
Action and Expression: Internalize Executive Functions. Ask students to recall strategies from the Warm-up that were useful. Invite students to identify one or two strategies they plan to continue with, and to share their plan with a partner before they begin. Supports accessibility for: Conceptual Processing, Memory
Launch
Groups of 2
Give each group a set of number cards, 2 recording sheets, and access to two-color counters and 10-frames.
“We are going to learn a new way to play Check It Off. This time, instead of adding to find each number on the recording sheet, you will subtract.”
Demonstrate choosing 2 number cards and display the numbers.
“Now I find the difference between the 2 numbers. The difference is the result when 1 number is subtracted from another. What is the difference between these 2 numbers? How do you know?”
Demonstrate checking off the difference on the recording sheet.
“After you check off the number, write a subtraction expression to show the difference.”
Demonstrate writing the subtraction expression.
“Continue taking turns with your partner. The person who checks off the most numbers wins.”
Activity
15 minutes: partner work time
As students work, consider asking:
“How did you find the difference?”
“Which expression did you write?”
Monitor for students who count out counters, remove some and count again, or count back to find the difference.
None
Student Response
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Activity Synthesis
“I have the numbers 7 and 3. How can I find the difference?”
Invite previously identified students to share.
“What subtraction expression can I write to show the difference?”
The purpose of this activity is for students to choose from activities that focus on counting up to 20 objects or adding and subtracting within 10. Students choose from any of the previously introduced stages of the listed centers. They are encouraged to choose the center that will be most helpful for them at this time.
Counting Collections
Number Race
Check It Off
Five in a Row: Addition and Subtraction
Launch
Groups of 2
“Now we are going to choose from centers we have already learned.”
Display the center choices in the student book.
“Think about what you would like to do first.”
30 seconds: quiet think time
Activity
Invite students to work at the center of their choice.
8 minutes: center work time
“Choose what you would like to do next.”
8 minutes: center work time
Choose a center.
Counting Collections
Number Race
Check It Off
Five in a Row: Addition and Subtraction
Student Response
None
Advancing Student Thinking
Activity Synthesis
“We have been playing a lot of games with addition and subtraction. What is something you have gotten better at doing from playing these games?”
Lesson Synthesis
Math Community
Display the math community chart and review the norms listed.
“Is there anything else we should add to our list of norms?”
Student Section Summary
We practiced counting collections.
We used different strategies to add and subtract.
We added 1 or 2 by counting on.
4 . . . 5, 6
The sum is 6.
We subtracted 1 or 2 by counting back.
6 . . . 5, 4
The difference is 4.
We played games with addition and subtraction within 10.
The sum is 7.
The difference is 3.
Standards Alignment
Building On
Addressing
1.OA.C.5
Relate counting to addition and subtraction (e.g., by counting on 2 to add 2).
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., ); decomposing a number leading to a ten (e.g., ); using the relationship between addition and subtraction (e.g., knowing that , one knows ); and creating equivalent but easier or known sums (e.g., adding by creating the known equivalent ).
